ARM: dts: ux500-skomer: Add KTD253 backlight

This enables the KTD253 backlight found on the Samsung
GT-S7710 "Skomer".

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200828223034.40983-1-linus.walleij@linaro.org